Hox genes and evolution of arthropod body plan: A comparitive analysis of direct targets of Ultrabithrox in Drosophila melanogaster and Apis mellifera

Abstract

Hox genes and the evolution of arthropod body plan: A comparative analysis of targets of Ultrabithroax in Apis mellifera and Drosophila melanogaster Amongst insects, there has been much diversity in the number and size of wings. Most insects have four wings (all directly contribute to the flight), while beetles and flies have only one pair of wings. It is now widely accepted that evolution at the level of a family of highly conserved (from insects to human) genes popularly known as Hox genes is one of the major factors for the diversity in animal body plan that we see now. Hox genes code for homeodomain containing proteins, which are expressed along the anterior posterior axis of the body and impart distinct identity to individual segments.Suppression of wing fate and specification of haltere fate in Drosophila by the Hox gene Ultrabithorax (Ubx) is one of the well-studied models for Hoxregulation of development. Earlier studies indicate that Ubx protein or its expression pattern has not evolved amongst the diverse insect groups, although they appear to regulate the differences between developing forewing and hindwing in at least three insect groups studied so far. This study is aimed to understand developmental and molecular events downstream of Ubx resulting in differencesin wing morphology between two divergent insect groups –Apis mellifera and Drosophila melanogaster. We used a genome wide approach to identify direct targets of Ubx in Apis and compared the same to those in Drosophila. While Apis and Drosophila lineages have diverged for more than 250 million years, large number of targets of Ubx are common between the two groups. Furthermore, the gene ontology pattern of targets of Ubx is largely similar between Drosophila and Apis. However, not all of them have detectable differences in the expression pattern between forewing and hind wing of Apis. Such targets have different topology of transcription/chromatin factor binding sites in their cis-regulatory regions compared to Drosophila.A large number oftargets ofUbxare exclusive to dipteran lineage- although at the ontology level, they belong to the same functional groups as targets that are common to the two species. Bringing them under the regulation of Ubx may have resulted in more drastic differences between forewing and hindwing in Drosophila as compared to Apis. Finally, we have functionally validated a gene that is a dipteran-specific target of Ubx and we show that it regulates Notch signaling during wing development and repression of its expression may be critical for maintaining haltere fate.